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as in the snakes. Among reptiles we find lizards, crocodiles, turtles, and snakes; and many extinct groups are known.

d. Aves, or birds, are characterised by the possession of wings and feathers, and have special modifications of the skeleton to aid in flight.

e. Mammalia. These are typically terrestrial animals. Their main characteristics are the possession of hair, the presence of two pairs of limbs, furnished with claws or hoofs, and the fact that they do not lay eggs, but give birth to young which are suckled by milk glands.

II. ACRANIA.-This group contains a number of lowly organised forms, possessing no skull, no distinct brain, and no limbs. Their sense-organs and other parts, such as the heart and liver, are in a very primitive and simple condition. They are interesting as telling us in which direction to look for ancestors, for they show us that these may be destitute of limbs, eyes, ears, or backbone, or even without any skeleton. I propose to leave this group for a time, and first consider in more detail the several groups of Craniota.

PISCES.-The characteristic mode of breathing in fish is by gills. Gill-clefts have no meaning apart from their respiratory function, yet they are present in the early stages of development of all Vertebrates without exception. They are one of the most characteristic features of Vertebrates, and have a constant relation to the heart, blood-vessels, nerves, muscles, and skeleton. In fish and some Amphibia they are preserved in a functional state throughout life.

The inevitable conclusion is that, of the five groups of Craniate Vertebrates, fish are the most primitive; and that the other four groups are descended, if not from fish, at any rate from gillbreathing forms, aquatic and presumably fish-like. The evidence afforded by Palæontology does not help us much on this point, the oldest known Vertebrates being fishes from the lower Silurian deposits. Geology would favour an aquatic origin for Vertebrates as for other forms, the instability of the land in comparison with the sea being well known.

AMPHIBIA. The chief point of interest in Amphibia is the well-known transitional series from the gill-breathing to the lung-breathing condition, which we have already dealt with at length when discussing the theory of change of function. Almost all Amphibia commence life as gill-breathers; but in one or two cases, such as Hylodes, where the eggs are not laid in water, owing to abundance of food-yolk the early stages are passed through before hatching, and, as in the higher Vertebrates, the gill-breathing stage is dropped out, though gill-clefts are developed (see Fig. 16).

The pedigree of Amphibia may be regarded as established, and is as follows: Amphibia are descended from fish, which migrated into rivers to avoid enemies and to obtain food. Owing to drought the air bladder became converted into a lung, and this at once conferred the power of going on land (see Fig. 29). This led to the conversion of a fin into a definite pentadactyle limb, a change the true nature of which is uncertain. The point of interest is, that every Amphibian repeats this history in its own development. Intermediate stages are not merely possible, but are known to actually occur at the present day, a most important point. The Axolotl is one of these, and is provided with large gills and also lungs; the Siren is similar, and is found in the swamps of the Southern States of North America; the mud-fish Lepidosiren, of Brazil, Protopterus of Africa, and Ceratodus of Queensland are other examples.

The origin of terrestrial Vertebrates is probably revealed to us in this way, and it must be considered that the fresh-water forms are descended from marine ancestors and the terrestrial forms from the fresh water.

REPTILIA. Reptiles are an exceedingly abundant group, both absolutely and relatively, in the secondary period: a great number of families and orders are extinct. Although their actual origin is uncertain, their descent from gill-breathing ancestors is proved by their passing through the gill-cleft stage in their development. No reptiles, however, have gills.

It is uncertain whether reptiles are derived from Amphibia, or descended from gill-breathing ancestors directly. The gigantic Ichthyosaurus (Fig. 30) and Plesiosaurus (Fig. 31), which have more than five digits in the hand and foot, have been supposed to indicate an independent origin from fishes, but are more probably a reversion to the paddle-like form in an aquatic group. Moreover, the oldest reptiles yet obtained as fossils are pentadactyl, and not provided with paddles. Of

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